Whether ATP or some other purine nucleotide plays a neurotransmitter role in the chick gut was studied. ATP, ADP and AMP produced a dose-dependent contraction of the isolated ileum. The effective concentration of 50% (EC50) of ATP was 20 times less than that of ADP and much less than that of AMP. ATP-induced contractions were completely resistant to blockade either by atropine or promethazine and were partially resistant to blockade by methysergide. Quinine blocked the effect of ATP, carbachol, 5-HT [serotonin] and histamine while caffeine blocked the effect of ATP only. Electrical stimulation of Remak's nerve resulted in contraction of the ileum which resembled that produced by ATP. The electrically-induced contraction was not affected by atropine, promethazine and methysergide but was blocked by either quinine or caffeine. ATP or a related compound may be acting as a chemical transmitter in Remak's nerve supplying the sympathetic innervation to the ileum of the chick.